1. Field of the Invention
The present invention relates to a method and unit for displaying images having multi-tone levels, in which a number of intermediate brightness levels are included, on a liquid crystal display (LCD), an AC type plasma display panel (PDP), or other bi-level display devices. More particularly, this invention pertains to the display of a number of intermediate tone levels based on basic tone levels using a dither method.
2Related Art Descriotion
Dither methods have been known as typical methods for generating signals for displaying half tone levels when images are displayed by a method involving mixed use of divided times and divided areas. Such methods are commonly used for displaying on an LCD whose picture elements can be only light or dark.
An explanation will now be given of the manner of displaying half tone levels when images are displayed by the method of divided times and divided areas. In this method, a typical picture element comprises a rectangular array, or group, of 2.times.2 dots (displaying cells) and each dot can display only a selected one of 2 brightness levels such as light and dark. The brightness of a picture element is changed according to a combination of light and dark dots, so that one picture element, constituted by 2.times.2 dots, can exhibit as many as 5 brightness levels. In general, the number of possible brightness levels of a multidot picture element is one greater than the number of dots.
In addition the brightness level of such picture element can also be changed by a combination of divided times or time division, since the states of the dots are changed to light or dark depending on the divided times. This method for displaying half tone levels using the divided times and divided areas, is usually called Frame Modulation, hereinafter FRM.
For example, on an LCD whose basic picture element consists of 3.times.2 dots, a 1/4 tone level is displayed as follows. In this example, one cycle of display on the LCD is divided into 8 time intervals so that a basic pattern is displayed periodically. As shown in FIG. 14 (a), for a basic pattern which is displayed at each time interval, one of the 2.times.2 dots is in its light state (represented by a black area) and the location of the dot which is in the light state is changed at each time interval. In order to maintain the quality of tone level display, as shown in the basic patterns of FIG. 14, the state of the dots in the next pattern should be changed to dark from light, as far as possible. For example, the location of the light dot in one picture element, can be shifted clockwise from one time interval to the next.
A 1/8 tone level, which is a lower tone level than, but an adjoining level to, the 1/4 (2/8) level, is displayed. by a train of displaying patterns Xb shown in FIG. 14 (b). The train Xb is generated based on the patterns of train Xa shown in, FIG. 14 (a), and a basic pattern of a totally dark picture element alternates with successive picture element patterns containing at least one light dot. By the above method, nine tone levels, i.e. 0/8 tone level to 8/8 tone level, can be displayed by a picture element having 2.times.2 dots when the FRM method with 8 time intervals is adopted.
Further tone levels may be displayed using the dither method and basic tone levels generated by FRM as explained above. For. example, a train of display patterns Ym shown in FIG. 14 (c), is generated from a combination of selected elements of trains Xa (FIG. 14 (a)) and Xb (FIG. 14 (b)) using a dither matrix (dither pattern) M which is shown in FIG. 15 (a). The dither matrix M shown in FIG. 15 (a) is a matrix of 2.times.2 dots and this matrix corresponds to a picture element. An `A` in the matrix M is a threshold value or a selecting parameter. The function of `A` assigned to a dot of matrix M is to select the state (light or dark) of the corresponding dot in the pattern of the train Xa, but to ignore the state of the corresponding dot in the pattern of the train Xb. The function of `B` assigned to the other dots of matrix M is to select the state of the corresponding dot in the pattern of the train Xb, but to ignore the state of the corresponding dot in the pattern of the train Xa. By the train Ym generated by the above method, a 3/16 tone level is displayed, and this tone level is, from a theoretical point of view, a middle tone level between the tone level 4/16 (=1/4) displayed by the train Xa and the tone level 2/16 (=1/8) displayed by the train Xb.
Another dither matrix N shown in FIG. 15 (b) is the inverse of the matrix M, and the locations of `A` and `B` are interchanged with those of matrix M. By this dither, matrix N, a train of display patterns Yn, shown in FIG. 14 (d), is generated and the a tone level displayed by this train Yn is also 3/16. However, in the train Yn, there are patterns which have two light dots, and patterns containing at least one light dot appear in alternation with dark patterns, i.e. without light dots. Therefore, the quality of a displayed image with the tone level of train Yn is poorer than that of the train Ym.
As illustrated in FIG. 16, if each display cycle is composed of, for example, 16 time intervals, 17 tone levels can be generated by FRM, and with the dither method explained above, 16 further dither tone levels will be newly generated. In order to maintain compatibility with the quantity of bits of IC used on the display unit, the number of tone levels is selected to be a power of 2. For example, in FIG. 16, 16 tone levels can be displayed by the trains of basic tone signals X.sub.) to X.sub.15 generated by FRM, except the train X.sub.16. And 16 tone levels can be displayed by the trains of dither tone signals Y.sub.0 to Y.sub.15. So, a total number 32 tone levels can be displayed.
Meanwhile, when the conventional dither method is used and images displayed by the method in which time division and area division are used, due to the following reasons, the maximum number of tone levels to be e displayed is limited to 32. The number of brightness levels of a picture element can be increased beyond 5 if picture elements are constituted by 3.times.3 dots or more. However, since this produces an increase in the dimensions of the picture element, image resolution and detail are reduced. In addition, a large number of different patterns will be displayed cyclically, so that the images will flicker. Hence, in order to get a fine image, the frame frequency of the displaying unit should be increased.